Mail processing systems for preparing mail pieces have long been well known and have enjoyed considerable commercial success. There are many different types of mail processing systems, including, for example, inserter systems that insert material into envelopes and mailing machines that print postage indicia on mail pieces. Optical sensors are commonly used in such mail processing systems to ensure that all parts in the system function in a coordinated way. For example, in a mail inserting system where a plurality of enclosure feeders are used to release documents onto a transport path and the released documents are collated into a stack for insertion into an envelope, optical sensors can be used to check the arrival of the envelope, the movement of the released documents, and so forth. Mailing machines range from relatively small units that handle only one mail piece at a time, to large, multi-functional units that can process thousands of mail pieces per hour in a continuous stream operation. The larger mailing machines often include different modules that automate the processes of producing mail pieces, each of which performs a different task on the mail piece. Such modules could include, for example, a singulating module, i.e., separating a stack of mail pieces such that the mail pieces are conveyed one at a time along a transport path, a moistening/sealing module, i.e., wetting and closing the glued flap of an envelope, a weighing module, and a metering module, i.e., applying evidence of postage to the mail piece. The exact configuration of the mailing machine is, of course, particular to the needs of the user. Optical sensors are commonly used to track the location of each mail piece as it passes through the machine via the transport mechanism. Based on the location of the mail piece, certain functions are initiated, e.g., weighing, printing, etc. Additionally, optical sensors can be utilized to determine if a mail piece has become jammed within the mailing machine.
As illustrated in FIG. 1A, an optical sensor 10, in general, comprises a photo-detector 12 and a light-emitting diode (LED) 14. When the optical sensor 10 is active, the LED 14 is activated to produce light to illuminate the photo-detector 12. When the photo-detector 12 is not blocked, the light from the LED 14 is detected by the photo-detector 12. In this state, the output voltage of the photo-detector 12 circuit is generally low. But when the photo-detector 12 is blocked by an object coming into the space between the LED 14 and the photo-detector 12, such as a mail piece 16 moving in the direction of arrow A, as illustrated in FIG. 1B, the light from the LED 14 does not illuminate the photo-detector 12, which causes the photo-detector 12 to be in a high resistive state. In this state, the output voltage of the photo-detector 12 circuit is generally high or substantially equal to the supply voltage. Alternatively, the output voltage of the photo-detector 12 could be opposite as described, depending on the circuit configuration, e.g., unblocked has a high output, and blocked has a low output. In general, the output voltage of a photo-detector 12 depends on the light that is detected by the photo-detector 12, which in turn depends on output of the LED 14, the distance of the photo-detector from the LED, the alignment between the photo-detector and the LED, and so forth. In order to ensure that the photo-detector 12 receives sufficient light when it is not blocked, the supply current to the LED 14 is usually set to a value that is higher than threshold value at which the photo-detector 12 would change the state of its output voltage.
Mail processing systems present a challenging environment for optical sensors. A long operating life combined with contamination such as paper dust build-up cause the operating characteristics of a sensor to deviate from the initial “brand-new” values. Optical sensors have to be designed with enough operating margin to overcome issues such as LED aging and dust build-up to ensure that the supply current to the LED is large enough such that sufficient light is detected by the photo-detector when not blocked. There are several conventional ways to achieve this, but each has problems. One method of achieving this is to increase the LED current. This, however, can reduce the sensor's ability to detect thinner material (as the light intensity may allow sufficient light to pass through thinner materials such that the optical detector detects enough light to remain in an “unblocked” state) as well as reduce the overall LED operating life. Another method to achieve this is to use detectors that are more sensitive. This, however, could lead to false triggers due to external light sources. Another method to achieve this is by the use of current amplifiers that require a digital to analog converter for each sensor. This, however, could significantly increase the cost of the machine, depending on the number of optical sensors utilized in the system.